Plasma atomic layer deposition

ABSTRACT

Plasma atomic layer deposition (ALD) is optimized through modulation of the gas residence time during an excited species phase, wherein activated reactant is supplied such as from a plasma. Reduced residence time increases the quality of the deposited layer, such as reducing wet etch rates, increasing index of refraction and/or reducing impurities in the layer. For example, dielectric layers, particularly silicon nitride films, formed from such optimized plasma ALD processes have low levels of impurities remaining from the silicon precursor.

REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.15/499,188, filed Apr. 27, 2017, which is a continuation of U.S.application Ser. No. 14/231,317 filed Mar. 31, 2014, now U.S. Pat. No.9,637,823, which are hereby incorporated by reference in their entirety.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

PARTIES OF JOINT RESEARCH AGREEMENT

The present invention is the result of a joint research agreementbetween ASM International, N.V. and Technische Universiteit Endhoven(Eindhoven University of Technology).

FIELD

The present invention relates generally to atomic layer deposition(ALD), and more particularly to identification and use of optimalconditions during plasma phases.

BACKGROUND

In recent years atomic layer deposition (ALD) has been adopted as amanufacturing technique in several fields, including the semiconductorindustry. ALD deposits films in a cyclical process, typicallyalternating exposure of the substrate to two or more reactants in phasesseparated in time and/or space, where each phase has a self-limitingeffect. For example, one phase of the cycle can chemically adsorb, in aself-limiting fashion, a monolayer or less of a precursor or fragmentthereof in each cycle. Often the adsorption is self-limited due toligands of the precursor being inert relative to the adsorbed species,such that after the substrate surface is saturated the adsorptionprocess stops. Reactants in a subsequent phase can react with theadsorbed species to remove the condition that limits the adsorption,such as stripping away ligands form the adsorbed species, for example bychemical reduction or replacement reactions, such that the precursor canagain adsorb in a self-limiting fashion in a subsequent cycle. In oneexample, less than a monolayer of an organic silicon precursor canadsorb in a first phase, and an oxygen-containing reactant can strip theorganic ligands from the adsorbed species and leave oxygen in a secondphase.

More complicated ALD recipes may include three, four or more reactants,and relative frequencies of the phases may be adjusted to tune thecomposition of the layer being formed. Typically, each cycle leavesabout a molecular monolayer or less per cycle. Many ALD processesaverage one monolayer every 3-10 cycles because of variety of reasons,such as steric hindrance from large precursor molecules prevents accessto all reaction sites in a single cycle, lack or low number of reactivesites or other reasons. Mutually reactive reactants can be keptseparated in time and/or space, e.g., by separating pulses by purging,or moving the substrate through different zones with separate reactants.However, variations on the process, such as schemes providing hybrid ALDand chemical vapor deposition (CVD) reactions, can obtain more than amonolayer per cycle.

Despite rather slow growth rates compared to traditional depositiontechniques, such as sputter deposition and CVD, ALD has been growing inpopularity for several reasons. For example, in the semiconductorindustry, ALD can provide much greater step coverage, or conformalgrowth, or smoother or more uniform films over complex topographycompared to other deposition techniques, particularly for very thinlayers over structures with high aspect ratios. Need for such conformallayers tends to increase as circuits become more dense. Because thetechnique is self-limiting in each cycle, and because usually the growthrate tends to be independent of small temperature variations over asubstrate, ALD offers almost perfect step coverage. Moreover, ALD tendsto involve lower temperatures than other deposition techniques, whichalso becomes more important with successive generations of integratedcircuits in order to conserve ever-stricter thermal budgets and preserveprecise device junction depths. Similarly, ALD is increasinglyattractive to other industries that could benefit from ultrathin,conformal and/or low temperature depositions.

To preserve self-limiting nature of ALD it can be important to preventthermal breakdown of precursors that are meant to adsorb largely intact.Thermal breakdown can lead to time-dependent CVD growth mechanisms whichcan nullify the conformal deposition advantages of ALD. At the sametime, some precursors demand significant energy to react with adsorbedspecies. Ensuring these competing conditions are satisfied can involvedelicate trade-offs between substrate temperatures and prolongedexposures to ensure saturative surface reactions.

Plasma ALD processes, sometimes referred to as plasma assisted ALD orplasma enhanced ALD (PEALD), have been developed in order to improve thereaction energy of some phases without increasing the temperature of thesubstrate. For example, an organic or halide reactant can adsorb lessthan a monolayer in a first phase, and a second phase can expose thesubstrate to the products of a nitrogen-, hydrogen- or oxygen-containingplasma to strip ligands from the adsorbed species and/or leave nitrogenor oxygen in the film. However, in some cases it has been difficult toobtain high quality films using conventional plasma ALD techniques.

SUMMARY

In one aspect, a method is provided for depositing a layer of siliconnitride by plasma atomic layer deposition. The method includes providinga substrate in a reaction space. In a first phase, the substrate iscontacted with a silicon precursor to adsorb an adsorbed species of thesilicon precursor on the substrate. In a second phase, the substrate iscontacted with excited nitrogen species to react with the adsorbedspecies. The excited nitrogen species is supplied to or formed in thereaction space for greater than about 0.1 s, and a residence time of thegas species during second phase in the reaction space is less than about1.0 s.

In another aspect, a method is provided for tuning a plasma atomic layerdeposition (ALD) process. The method includes conducting a plasma ALDprocess to deposit a layer. The plasma ALD process has an adsorptionphase and a plasma phase. A quality of the layer is measured. Aresidence time for the plasma phase is modulated to form a modifiedversion of the plasma ALD process. The modified version of the ALDprocess is conducted to deposit a further layer, and the quality of thefurther layer is measured.

In another aspect, a method of optimizing a plasma ALD process isprovided. The method includes optimizing impurity levels in filmsdeposited by the plasma ALD process by modulating residence time forexcited species in a reaction space volume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a general process flow for simpleplasma ALD sequences.

FIGS. 2A-2D are schematic representations of examples of remote plasmareactors that can be employed for plasma ALD processes described herein.

FIG. 3 is a schematic representation of a direct plasma reactor that canbe employed for plasma ALD processes described herein.

FIG. 4 is a flow chart illustrating techniques for optimizing filmquality and/or growth rates for plasma ALD processes.

FIG. 5 is a graph illustrating deposition rates as a function ofresidence time for the plasma phase of a plasma ALD process.

FIG. 6 is a graph illustrating deposition rates as a function of plasmaphase duration at three different residence times for the plasma phaseof a plasma ALD process.

FIG. 7 is a graph illustrating wet etch rates as a function of residencetime for the plasma phase of a plasma ALD process.

FIGS. 8A and 8B are scanning electron micrographs (SEMs) of filmsdeposited using two different residence times for the plasma phase of aplasma ALD process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted in the Background section above, despite the theoreticaladvantages that plasma ALD provide in lowering substrate temperatures,thereby preserving thermal budgets and opening the door to precursorsthat might decompose at higher temperatures, it has been difficult toobtain high quality films, particularly dielectric films such as siliconoxides, metal oxides and silicon nitrides.

Somewhat counterintuitively, the inventors have found that in plasma ALDof certain materials, limiting the gases' residence time in the reactionspace for each plasma phase can improve the quality of the film,indicating a fuller reaction can be achieved with lower exposure time tothe excited species. Herein reference is made to excited species' orplasma products' residence time for convenience. It will be understood,however, that the residence time applies to all vapor phase speciessupplied through the reaction space during the plasma phase, includingany non-excited species. Conventionally, ALD designers have tended toprolong reactant exposure times to achieve saturation of surfacereactions, or modulated other parameters, such as temperature, pressureor plasma power, in attempts to optimize deposition speed and quality.Moreover, a prolonged exposure to reactants would be expected to lead tomore efficient saturation with the least amount of reactant. However,conventional attempts at optimization have not been satisfactory formany plasma ALD processes, particularly for dielectric oxides andnitrides. For example, depositing silicon nitride from alternation oforganic precursors with nitrogen plasma products has been found toproduce poor film density, wet etch rates and/or impurity levels.According to the teachings herein, reducing plasma reactant residencetime can increase film quality, and modulating residence time againstmeasured film quality can improve film quality (e.g., low impuritylevels, high density).

FIG. 1 is a simplified flow diagram of a generic plasma ALD process, towhich techniques described herein can be applied. For purposes ofillustration, a simple two-reactant cycle is illustrated. The skilledartisan will readily appreciate that each cycle can include additional,unillustrated pulses of different reactants (for example, a thirdreactant pulse, fourth reactant pulse, etc., can be introduced), or ofthe same reactants, and that not all cycles need to be identical (e.g.,a third reactant can be introduced every five cycles to incorporate adesired percentage of different elements). The illustrated two-reactantcycle suffices to explain the embodiments, but the skilled artisan willappreciate that the principles and advantages of the embodiments taughtherein can be readily extended to more complex ALD recipes. Furthermore,because it is cyclical, the plasma ALD process need not begin with anadsorption phase and can begin with an excited species phase. Prior toloading in to a plasma deposition reactor, the substrate may bepre-cleaned. Additionally or alternatively, the substrate may be cleanedor otherwise conditioned for deposition prior to the process illustratedin FIG. 1.

For convenience of description, the reactant that is adsorbed in theplasma ALD process is referred to herein as a precursor, while theexcited species is referred to as a reactant. It will be understood thatboth the precursor and the excited species serve as ALD reactants, andalso that the excited species can also serve as a precursor to theextent it leaves elements in the deposited film. In dielectric filmdeposition examples, excited nitrogen or excited oxygen species canperform exchange reactions with ligands from a prior adsorbed layer.Also for convenience, the substrate is considered to include previouslydeposited or adsorbed materials, such that references to contacting thesubstrate will be understood to include contacting previously depositedor adsorbed materials.

In the first illustrated block 10, the first reactant, or precursor,contacts a substrate to adsorb thereon. It will be understood that theadsorbed species may comprise a fragment of the precursor molecule, suchthat some ligands may be lost in the process, but that the precursor isnot thermally decomposed. In the next block 20, any excess precursor isremoved from the reaction space Typically supply of the precursor isstopped at this stage. Such removal can be accomplished, e.g., bypurging the reaction chamber or space, pump down, moving the substrateaway from a zone in which the precursor is supplied, or combinationsthereof. It will be understood that removal of reactants from thereaction space can be a relative action and can be understood asmovement of excess reactants in relation to the vicinity of thesubstrate; for example, in space-divided ALD moving the substrate fromthe zone effectively moves the reaction space associated with thesubstrate, leaving the reactants of the prior phase behind in adifferent reaction zone. In the next block 30, the substrate iscontacted with excited species, which react with the adsorbed species onthe substrate. The phase of plasma ALD in which excited species aresupplied can be referred to as a “plasma phase,” although plasmadischarge is not always employed to create the excited species, and evenwhen an actual plasma discharge is created, it may be remote from thesubstrate and plasma products are supplied to a downstream substrate.For simplicity, the term “excited species” used herein can be understoodto encompass electrons, ions, radicals, atoms or other excited speciesthat can, for example, be generated by a plasma. Typically the excitedspecies are formed in a plasma discharge and depending on how they aresupplied to the reaction space where substrate is loaded the excitedspecies may comprise electrons, ions, radicals and/or atoms, for exampleoxygen, hydrogen or nitrogen plasma, ions, radicals, atomicoxygen/hydrogen/nitrogen, or other excited species that can, forexample, be generated by a plasma. Thus some excited species such as O,H and N can be electrically neutral. In some embodiments the excitedspecies does not comprise hydrogen. In some embodiments the excitedspecies comprise only nitrogen. In some embodiments the excited speciescomprise only oxygen. Skilled artisan will understand that, for example,during plasma discharge additional gases may be present in addition tothe excited species, such as rare gases like He, Ne, Ar to help inplasma ignition or otherwise make the process better like more stableand repeatable, but these species are considered not to be significantcontributors to the film growth.

Excited species may be generated in situ in the reaction chamber, or maybe supplied from a remote plasma unit (RPU) or the like. In variousembodiments excited species may be generated by coupling power, such asby RF alternated electrical fields, microwave standing waves,ultraviolet light, or other forms of energy, to a flowing vapor phasereactant. The excited species reaction with the adsorbed species maystrip ligands from adsorbed species to prepare the surface forsubsequent reactants, and may additionally contribute elements to thefilm. In block 40, excess excited species are removed from the reactionspace. Such removal can be accomplished, e.g., by turning off the powerthat generates the excited species, and by physical removal as in block20, e.g., by purging, pump down, moving the substrate away from a zonein which the excited species is supplied, or combinations thereof. Insome embodiments, the excited species is formed from gases that areinert under the deposition conditions in the absence of excitement, suchas N₂ or O₂, depending upon the reactivity of the adsorbed species.Limiting residence time as disclosed below aids in more quicklyeliminating active species in block 40 after the plasma power isremoved.

The blocks 10-40 together define a cycle 50, which can be repeated untila film of the desired thickness is left on the substrate. As notedabove, the cycle 50 can include additional reactant supply blocks (notshown), and need not be identically performed in each repetition. Forexample, other reactant(s) can be introduced in separate phase(s) ineach cycle; other reactant(s) can be introduced in separate phase(s)every few cycles to incorporate a controlled amount of another element;other reactant(s) can be introduced in separate phase(s) in each cycleor every few cycles for gettering; separate reactants can be providedfor stripping ligands from the adsorbed species and for contributingadditional elements; etc.

The techniques taught herein can be applied to plasma enhanced orassisted ALD in a wide variety of reactor configurations. FIGS. 2A-2D,for example, illustrate a number of remote plasma ALD reactors. FIG. 3illustrates a direct plasma ALD reactor. Similar parts are referred toby like reference numbers in the drawings.

FIG. 2A illustrates a showerhead ALD reaction chamber 200 with a remoteplasma unit (RPU) 202. The reactor includes a showerhead 204 configuredto receive and distribute reactant across a substrate 206 on a substratesupport 208. A reaction space 209 is defined between the showerhead 204and the substrate 206. A first inlet 210 communicates with a source of afirst reactant, or precursor, and a second inlet 212 communicates withthe RPU 202, which in turn communicates with a source of a secondreactant to be activated or excited in the RPU 202. Additional inlets(not shown) can be provided for separate sources of inert gases and/oradditional reactants, and the showerhead 204 can also be provided with aseparate exhaust (not shown) to speed removal of reactants betweenphases. While the inlet 210 for a non-plasma reactant and the inlet 212from the RPU are both shown communicating with the plenum of theshowerhead 204, it will be understood that in other arrangements theinlets can independently feed reactants to the reaction space and neednot share a showerhead plenum. An exhaust outlet 214, shown in the formof an exhaust ring surrounding the base of the substrate support 208,communicates with a vacuum pump 216.

FIG. 2B illustrates a different configuration of a plasma ALD reactionchamber 220. The operation of reaction chamber 220 can be similar tothat of the showerhead reaction chamber 210 of FIG. 2C, except that noshowerhead plate is present. The arrangement lacks the uniformdistribution of a showerhead plate. Such distribution can be dispensedwith for the theoretically self-saturating surface reactions of ALD,although with less uniform distribution each phase may take longer tosaturate the substrate 206. An advantage of the chamber 220 of FIG. 2Bis that no restrictions are present between the inlet 212 for the remoteplasma products from the RPU 202, such that more active species surviveto reach the substrate 206 compared to use of an intervening showerheadplate. Such line-of-sight remote plasma arrangements can be advantageousfor processes that benefit from more energetic active species during theplasma products phase of the plasma ALD process, or processes thatemploy activated species more susceptible to recombination. Theillustrated chamber 220 includes a separate inlet 210 to the reactionspace 209 for non-plasma reactants, also referred to herein asprecursors, to minimize deposition reactions upstream of the reactionspace 209.

FIG. 2C illustrates a different configuration of a plasma ALD reactionchamber 230. Typically known as a horizontal flow reactor, the reactionchamber 230 is configured with a first reactant inlet 210 and a secondreactant inlet 212, communicating with the RPU 202, at one side of thechamber 230, and an exhaust outlet 214 at an opposite side of thereaction chamber 230. As noted with respect to FIG. 2C, additionalinlets (not shown) can be provided for separate sources of inert gasesand/or additional reactants. Separate inlets 210 and 212 are shown tominimize deposition reactions upstream of the reaction space 209, as isgenerally preferred for ALD reactors, but it will be understood that inother arrangements the different reactants can be provided through acommon inlet manifold. The skilled artisan will appreciate that in othertypes of horizontal flow reactors, the different reactants can also beprovided from different sides of the chamber, with separate exhausts onopposite sides, such that a first reactant can flow in one direction anda second reactant can flow in another direction in separate phases.

FIG. 2D illustrates another example of a remote plasma ALD reactionchamber 240. The illustrated chamber is configured for space-divided ALDreactions, rather than time-divided reactions. The space-dividedreactions employ different zones, here zones A, B, C and D, throughwhich substrates move. Alternatively, the gas injection system can movein relation to the substrates and substrates might be stationary orrotating. The zones are separated by barriers 242, which may be physicalwalls, inert gas curtains, exhausts, or combinations thereof thatminimize vapor interactions among the zones A-D. The substrate support208 can take the form of a rotating platform, as shown, or a conveyorbelt (not shown) for linearly arrayed zones. In other example, zone Acould be supplied consistently with a first reactant, such as anon-plasma precursor that adsorbs on the substrate, zones B and D couldbe supplied with inert or purge gas, and zone C could be supplied with aremote plasma activated second reactant. Substrates 206 (four shown)move through the zones to sequentially be exposed to the first reactant(zone A), inert gas (zone B), remote plasma activated second reactant(zone C), and inert gas (zone D) before the cycle is repeated. In thecase of space-divided plasma ALD, the residence time of the reactantscan depend on both the speed of the reactants through the zone as wellas the rate of movement of the substrate support 208. In some cases thesubstrate is stationary or rotating and the gas supply system, such asgas injector(s), is rotated over the substrates. Rotation speed of theinjector(s) or substrates can also affect the gas residence time. Invariations on space-divided ALD, a combination of space-divided andtime-divided ALD could supply different reactants at different times tothe same zone, while substrates move through the zones. Each zone maysupply separate reactants, and additional zones may be added byproviding larger platforms divided by greater numbers of zones, or byproviding longer conveyors through greater numbers of zones.

FIG. 3 illustrates a direct plasma ALD reaction chamber 300, in which aplasma 310 is formed directly over the substrate 206 within the reactionspace 209. For example, potential difference across two electrodes 315,320 is alternated at radio frequency (RF) to generate alternating fieldsin the reaction space, which in turn generates a plasma discharge fromvapors supplied to the reaction chamber. In the illustrated example, agrounded electrode 315 is provided within the substrate support 208,while a powered electrode 320 is spaced above the substrate 208 andconnected to an RF power source 325. In other arrangements, the poweredelectrode can be in the substrate support and/or the walls of thechamber. The skilled artisan will appreciate that power may be coupledto the reactant gases to generate excited species within the reactionspace in other ways, such as by inductive coupling from coils outsidethe chamber. Reactants for a direct plasma reactor can be supplied inany suitable manner, including the inlet arrangements of thetime-divided ALD reactors of FIGS. 2A-2C, or the space-divided ALDreactor of FIG. 2D. For example, the powered electrode may be ashowerhead through which at least one of the reactants is supplied.Separate inlets may be provided for different reactants, or multiplereactants can be supplied through the same inlet(s). Because at leastone of the reactants may be inert (relative to the other reactant(s))until activated by the plasma in the reaction space 209, supplyingmultiple reactants sequentially through the same inlet will notnecessarily cause deposition reactions to take place in the inlet.

In general plasma ALD reactors can be configured for single substratesor for multiple substrates. In some arrangements, multiple substratesare stacked vertically; however, in order to maximize effectiveness ofthe excited species, multiple substrate reactors preferably array thesubstrates adjacent to one another, similar to the enlarged platformarrangement of FIG. 2D, whether or not configured for space-divided ALDoperation. The effective reaction space volume can range from about 0.3liter to about 20 liters, preferably about 0.5 liter to about 15 liters,and more preferably about 1 liter to about 10 liters and most preferablybetween about 1.0 liters to about 5 liters The reactor can be configuredto accommodate standard semiconductor substrates, such as 150 mm, 200mm, 300 mm or 450 mm wafers.

FIG. 4 illustrates a process 400 for optimizing a plasma ALD process. Anestablished plasma ALD process, such as the simple two-reactant processillustrated in FIG. 1 or any of the more complex variations discussedwith respect to FIG. 1, can be employed as a starting point fordepositing a layer as shown in block 410. As aspect of the film orprocess quality is measured at block 415. Film quality can be measured,for example, using the level of impurities (e.g., oxygen or carbon for asilicon nitride deposition recipe), or an indicator of density orstoichiometry (e.g., etch rate for given etch conditions or index ofrefraction). The deposition recipe is then altered by modulation ofresidence time for the plasma phase, in which the substrate is exposedto excited species, at block 420, and the process repeated, as indicatedby loop 425, with the new parameters. In this way the process recipe isadjusted to maximize film quality as measured, for example, by density,wet etch rates, index of refraction or impurity levels.

As noted above, the skilled artisan may expect an ALD process to beoptimized by altering conditions to more efficiently achieve saturationof surface reactions. For example, if an unacceptably low growth rateper cycle or high impurity level is found, more efficient reactions maybe expected from, e.g., prolonging a reactant exposure step orincreasing substrate temperatures, as long as temperature remain insuitable regime (e.g., below the thermal decomposition temperature ofreactants). Thus, conventional parameters modulated include temperature,pressure, reactant supply rates and reactant exposure durations. Inplasma processing, plasma power may also be modulated.

It has been found that, for plasma phases of plasma ALD processes,modulation of reactant residence time in the process chamber can have asignificant impact on film quality for plasma ALD. Without being limitedby theory, it is possible that the more reactive species derived fromplasma or otherwise excited can cause interactions within the reactionspace that lead to redeposited species. For example, it is possible thatligands removed from the adsorbed species of a prior phase can interactwith the energetic species still in the reaction space, can form newspecies, which can then interact with the substrate surface again andoccupy reaction sites that would otherwise be available for the next ALDphase. Such redeposition can cause the process to take longer to removeligands and byproduct from the substrate surface. Modulation ofresidence time, particularly reducing residence time by increasing thespeed of the plasma products' movement through the reaction space, andlimit the potential for redeposition and obtain more complete surfacereactions, leaving fewer undesired impurities from unremoved orredeposited ligands and/or more dense films.

The residence time τ is defined as

$\begin{matrix}{\tau = \frac{V}{S}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where V is the volume of the reaction space and S is the effectivevolumetric pumping speed for the vacuum pump exhausting the reactionspace. The residence time τ is an indication of how long the relevantspecies spend inside the reactor before being removed to the exhaust. Itrepresents the 1/e time, or average time for reduction of the relevantspecies in the reaction space by 63.2%, rather than an absolute limitfor every molecule of the relevant species.

The residence time can be varied by varying the effective pumping speed,where the volume of the reactor can be considered fixed. The effectivepumping speed can be varied by changing the pump or its operatingconditions. For example, the rotation speed of pumps can be varied, suchas rotation speed of turbo pumps. The effective pumping speed alsodepends on the conductance of the pump line so that is also a hardwareadjustment that can be used to change the effective pumping speed.Residence time can also be modulated by controlling valves between thereactor and pump, e.g. by partly closing an exhaust valve partly or byusing a butterfly valve, and can be adjusted through the operatingsoftware of the reactor. The effective pumping speed can also dependupon the pressure inside the reactor so by changing the pressure, forexample by changing the overall gas inlet flows, the effective pumpingspeed can also be varied. Moreover, different gases in the gas mixturecan exhibit different effective pumping speeds where all otherparameters are kept constant. The skilled artisan can determine howchanging such additional variables can affect the effective pumpingspeed S for calculation of the residence time.

In some embodiments, residence time for the excited species in theplasma phase of the plasma ALD process can be optimized to minimizeimpurities in the deposited film, or to maximize film density. In someembodiments, impurity levels are kept below about 5 atomic %, preferablybelow about 2 atomic %, and more preferably below about 1%. In someexamples, optimization can be performed using the principles taughtherein to achieve impurity levels below about 0.5 atomic %. “Impurities”refer to elements present in the ALD reactants (both plasma andnon-plasma reactants) that are not desired in the film, and thepercentages provided above represent total impurity percentages in thedeposited film, and therefore can also represent maximum impurity levelsfor each contaminant present in the precursors. For all materialshydrogen can be excluded from the definition of impurities as it issomewhat difficult to analyze accurately for very thin films. Forexample, for silicon oxide or metal oxides, impurities to be minimizedinclude halides, carbon and nitrogen (to the extent these elements arepresent in the reactants). For silicon nitride or metal nitrides,impurities to be minimized include halides, carbon and oxygen (to theextent these elements are present in the reactants).\

In other arrangements, residence time for the excited species in theplasma phase of the plasma ALD process can be optimized to maximize anindicator of film density. The nitrogen plasma residence time forsilicon nitride plasma ALD recipes in the examples below, for example,can be optimized to produce films with index of refractions greater thanabout 1.9, and preferably greater than about 1.95. Similarly, thenitrogen plasma residence time for silicon nitride plasma ALD can beoptimized to produce films with low wet etch rates, for example, wetetch rates in a buffered, 7:1 dilute HF solution can be less than about1.5 nm/min, preferably less than about 1.0 nm/min and more preferablyless than about 0.7 nm/min.

At the same, in keeping with general advantages for ALD, plasma ALDprocesses with minimized residence time, as taught herein affords highlyconformal layers over topography, relative to non-ALD depositiontechniques. Conformality is typically measured in terms of step coverageover features on the substrate, and can either be measured as a ratio ofdeposited thickness on sidewalls to deposited thickness on top surfaces(S/T), or as a ratio of deposited thickness on bottom surfaces todeposited thickness on top surfaces (B/T). Under either measure, theprocesses taught herein can afford step coverage of more than about 75%,preferably more than about 90%, more preferably more than about 95% andin some cases more than about 99%. These ratios can hold true oversubstrates with features having height-to-width aspect ratios (AR) ofabout 2:1 or greater, preferably about 5:1 or greater, and even about8:1 and greater.

In some embodiments, residence time for the excited species in thereaction space is less than about 1.0 s. Preferably the residence timeis less than 0.5 s, more preferably less than about 0.2 s. In someembodiments, residence time may be below about 0.05 s and if carefullyoptimized residence times below about 0.01 s can be achieved. Dependingon the reaction space volume, the effective pumping speed to accomplishsuch residence times can vary. For most single substrate reactors, ormulti-substrate reactors with zones sizes for individual substrates(similar to FIG. 2D), pump speed can vary from about 25 m³/h to about5000 m³/h. Preferably the pump speed is between about 50 m³/h and about2500 m³/h, and more preferably between about 100 m³/h and about 2000m³/h. Similarly, pressure within the reaction space can vary dependingupon the reactor design and gas mixtures, but typically pressures can bebetween about 10⁻³ mbar and about 10³ mbar. Preferably pressure isbetween about 10⁻² mbar and about 20 mbar, more preferably between about10⁻¹ mbar and about 10 mbar.

While the residence time for the excited species is kept relatively low,representing a relative rapid rate of flow through the reaction space,the duration of the plasma phase is desirably kept long enough to ensuresaturative reaction of the excited species with the substrate surface.The plasma phase duration is the time during which the excited speciesis supplied to the substrate while plasma or other exciting power iscoupled to the gas from which the excited species is generated. In someembodiments, the plasma phase can have a duration greater than about0.05 s. Preferably the plasma phase has a duration greater than about0.1 s, more preferably greater than about 0.2 s or 0.5 s. Whilelengthening the cycle duration may not be economical in terms ofthroughput, higher quality films can be more important in somesituations, particularly where very thin, conformal layers are desired.However, most reactors will have practical limits on the duration of theplasma phase, such as the need to avoid overheating the substratesand/or equipment. Typically single substrate plasma ALD reactors employplasma phase durations of less than about 10 s.

The teachings herein with respect to residence time can be applied to anumber of different plasma ALD recipes, including recipes in whichexcited nitrogen, hydrogen or oxygen species are provided in the plasmaphase(s) of the plasma ALD process. Specific examples are provided belowfor silicon nitride (SiN). Examples of insulating films include SiN,silicon oxide (SiO), mixtures of SiN and SiO (SiON), aluminum nitride(AlN), etc., where the above abbreviations encompass a wide variety ofatomic ratios and do not necessarily represent stoichiometric materials.Other examples of insulating films include high-k films such as ZrO₂,HfO₂, Al₂O₃, Ta₂O₅, etc., as well as mixtures and non-stoichiometricforms of the above. The principles taught herein can also be applied toconductive films, such as transition metal nitrides, carbides,carbonitrides, borides and mixtures thereof, where plasma phases cansupply excited nitrogen, hydrogen, carbon and/or boron species.Furthermore, excited oxygen species are also useful for conductiveoxides such as RuO₂, IrO₂, SnO₂, etc.

FIGS. 5-8B reflect the results of experiments performed for depositionof a dielectric, particularly silicon nitride, in a reactor having theconfiguration of FIG. 2B, commercially available under the trade nameFlexAL® from Oxford Instruments plc of Abingdon, UK. For the datapresented herein, and for typical reactors, the reaction space volume Vemployed for the calculation of τ includes the reaction chamber from theplasma product inlet 212 to the exhaust 214. For chamber configurationswith unusually high chamber volume downstream of the substrate andupstream of the exhaust outlet (e.g., greater than 30% of the totalchamber volume), the calculation of τ can employ an “effective” reactionspace volume that excludes the volume of the reaction chamber downstreamof the substrate. Effective reaction space volumes can be as discussedabove with respect to the various reaction configurations.

In the experiments, silicon nitride was deposited by a first phaseincluding exposing the substrate to an organic silicon precursor thatadsorbs on the substrate surface; followed by stopping provision of thesilicon precursor to the substrate; followed by a second phase includingexposing the substrate to excited nitrogen species that reacted with theadsorbed silicon species from the first phase; followed by stoppingprovision of the silicon precursor. It will be understood that, becausethe plasma ALD process is cyclical, the “first” phase need not be thefirst step in the process. In the experiments, stopping the provision ofa reactant included both stopping the flow of the reactant to thereaction space and removal of excess reactant from the substrate surfaceby provision of inert purge gas through the reaction space. In the caseof the plasma phase, the inert purge gas can be a continuation of N₂ gasafter plasma power is removed from the flow, and purge gas canadditionally be supplied through the inlet for the silicon precursor. Inspace-divided ALD embodiments, stopping the provision of a reactant caninvolve moving the substrate out of the silicon precursor zone, andmoving the substrate through a flowing inert gas (e.g., a purge gascurtain or another zone supplied with purge gas) can additionally removeexcess excited species from the substrate.

The excited nitrogen species can be provided by coupling power to anitrogen source gas alone (especially N₂), a nitrogen source andhydrogen (e.g., N₂+H₂), or a nitrogen source and noble gas forsupporting plasma generation (e.g., NH₃+Ar, He or Ne). Preferablynitrogen plasma includes supply of N₂ without noble gases.

Preferably, the silicon precursor is an aminosilane or an aminesilane.In some embodiments the silicon precursor comprises a silicon amine,where silicon is bonded to two nitrogen atoms and two hydrogen atoms.For example, the silicon precursor may comprise bis(dialkylamine)silane,(R₂N)₂Si—H₂. In some embodiments the silicon precursor comprises BDEAS(=bis(diethylamino)silane). In some embodiments the silicon precursorcomprises a silicon amine, where silicon is bonded to three nitrogenatoms and one hydrogen atom. For example, the silicon precursor maycomprise tris(dialkylamine)silane, (R₂N)₃Si-H₁. In some embodiments, thesilicon precursor comprises a silicon amine, where silicon is bonded tofour nitrogen atoms. For example, the silicon precursor may comprisetetrakis(dialkylamine)silane, (R₂N)₄Si. In some embodiments, the siliconprecursor comprises aminosilane, where the silicon is bonded to onenitrogen atom and three hydrogen atoms. For example, the siliconprecursor may comprise dialkylaminesilane, (R₂N)Si—H₃.

Organic compounds having a Si—Si bond and an NH_(x) group eitherattached directly to silicon (to one or more silicon atoms) or to acarbon chain attached to silicon are used in some embodiments. In someembodiments, the silicon precursor may comprise an aminodisilane, suchas hexakis(ethylamino)disilane. In some embodiments the silicon compoundmay have the formula:

R^(III) _(3-x)(R^(II)R^(I)N)_(x)Si—Si(N—R^(I)R^(II))_(y)R^(III) _(3-y)

wherein:

-   -   x is selected from 1 to 3;    -   y is selected from 1 to 3;    -   R^(I) is selected from the group consisting of hydrogen, alkyl,        and substituted alkyl;    -   R^(II) is selected from the group consisting of alkyl and        substituted alkyl; and    -   R^(III) is selected from the group consisting of hydrogen,        hydroxide (—OH), amino (—NH₂), alkoxy, alkyl, and substituted        alkyl; and    -   each of x, y, R^(III), R^(II) and R^(I) can be selected        independently from each other.

In some embodiments the silicon compound ishexakis(monoalkylamino)disilane:

(R^(II)—NH)₃Si—Si(NH—R^(II))₃

In other embodiments the silicon compound is (CH₃—O)₃Si—Si(O—CH₃)₃

In still other embodiments, the silicon precursor is a halide, such as asilicon chloride. Examples include but are not limited tooctachlorotrisilane (Si₃Cl₈, OCTS); hexachlorodisilane (Si₂Cl₆, HCDS);and dichlorosilane (SiH₂Cl₂, DCS).

In the experiments, a silicon amine, and particularly SiH₂(NH^(t)Bu)₂,also referred to as BTBAS, was employed in alternation with products ofa nitrogen-containing plasma, particularly a plasma generated from N₂gas. A variety of plasma ALD conditions were applied, includingsubstrate temperatures from about 100° C. to about 500° C., chamberpressures from about 10 mTorr to about 100 mTorr, and plasma exposuretimes from about 2 seconds to about 20 seconds.

FIG. 5 is a graph illustrating deposition rates as a function ofresidence time for the plasma phase of a plasma ALD process in whichBTBAS was alternated with N₂ remote plasma to form SiN. At threedifferent flow rates of N₂ at a substrate temperature of 200° C.,shorter residence times for gases in the reaction space during theplasma phase led to decreased deposition rates in terms of growth percycle, as shown.

FIG. 6 is a graph illustrating deposition rates as a function of plasmaphase duration at three different residence times for the plasma phaseof a plasma ALD process in which BTBAS was alternated with N₂ remoteplasma to form SiN. Each of the three experiments reflected an initialrapid increase in deposition rates as the duration of the plasma phasewas increased, peaking at 2-3 seconds, beyond which further prolongedplasma phases reduced the rate of deposition in terms of growth percycle. The data reinforces the findings of FIG. 5 that reducing theresidence time by increasing the speed of flow through the reactionspace reduced the deposition rate.

FIG. 7 is a graph illustrating wet etch rates as a function of residencetime for the plasma phase of a plasma ALD process in which BTBAS wasalternated with N₂ remote plasma to form SiN. Each of the films was etchin a 7:1 buffered HF solution and the rate of etch measured. Lower wetetch rates are an indication of higher quality for the SiN films. Asshown, lower residence time correlates with lower wet etch rates andthus higher quality SiN films. The depositions employed substratetemperatures of 400° C. with a long (10 s) plasma phase. The residencetime for the excited species was modulated by controlling the chamberpressure and the N₂ flow rate. Against expectations, even at highpressures (80 mTorr and high flow) very good quality SiN can beobtained.

FIGS. 8A and 8B are scanning electron micrographs (SEMs) of filmsdeposited using two different residence times for the plasma phase of aplasma ALD process in which BTBAS was alternated with N₂ remote plasmato form SiN. The layers were deposited under two different sets ofconditions, with FIG. 8A representing a low residence time and lowpressure (τ=0.35 s; P=24 mTorr) and FIG. 8B representing a highresidence time and high pressure (τ=1.2 s; P=80 mTorr). The same tworecipes were employed to deposit on features with different aspectratios of 1.2, 2 and 4. The higher residence time employed for theprocess used for FIG. 8B demonstrated better conformality, likely due toa higher frequency of particle collisions under those conditions, suchthat the flux of the excited species was more isotropic. In particular,Table I below illustrates the sidewall/top (S/T) step coverage and thebottom/top (B/T) step coverage for both recipes on all three featuretypes.

TABLE I τ = 0.35 s; τ = 1.2 s; P = 24 mTorr P = 80 mTorr Aspect ratioS/T B/T S/T B/T 1.2 93% 95% 95% 95% 2 80% 94% 93% 95% 4 60% 93% 75% 90%

However, the shorter residence time for the gases during the plasmaphase in the process used for FIG. 8A achieved a better quality SiN filmcompared to the longer residence time used for FIG. 8B. In particular,the shorter residence time during the plasma phase led to asignificantly lower wet etch rate compared to the film deposited withthe longer residence time during the excited species phase, and etchingwas performed uniformly for layers deposited on the smaller aspect ratiofeatures. Moreover, conformality achieved using shorter residence times,while not as high as longer residence times provided, remain much higherthan alternative, non-ALD deposition techniques and it is expected thatfurther optimization can yield still better conformality whilemaintaining the high quality layers taught herein.

Although certain embodiments and examples have been discussed, it willbe understood by those skilled in the art that the scope of the claimsextend beyond the specifically disclosed embodiments to otheralternative embodiments and/or uses and obvious modifications andequivalents thereof.

What is claimed is:
 1. A method of depositing a layer of dielectricsilicon nitride by plasma atomic layer deposition, the methodcomprising: providing a substrate in a reaction space; in a first phase,contacting the substrate with a silicon precursor to adsorb an adsorbedspecies of the silicon precursor on the substrate; and in a secondphase, contacting the substrate with excited nitrogen species to reactwith the adsorbed species, wherein the residence time of the excitedspecies in the reaction space during the second phase is less than about1.0 s, and cyclically repeating the first phase and second phase to forma dielectric SiN layer of a desired thickness.
 2. The method of claim 1,wherein the excited nitrogen species is supplied to or formed in thereaction space for greater than about 0.1 s.
 3. The method of claim 1,further comprising purging the reaction space between the first phaseand the second phase.
 4. The method of claim 1, wherein in the secondphase the excited nitrogen species replace the ligands with nitrogen toleave a layer of dielectric SiN.
 5. The method of claim 1, wherein thesilicon precursor is organic, and the layer of dielectric SiN containsless than 2 atomic % carbon.
 6. The method of claim 1, wherein thesilicon precursor is a halide and the layer of dielectric SiN containsless than about 2 atomic % of a halogen from the silicon precursor. 7.The method of claim 1, wherein the silicon precursor comprises a siliconamine.
 8. The method of claim 7, wherein the silicon precursor comprisesBTBAS.
 9. The method of claim 1, wherein in the second phase the excitednitrogen species are supplied from a remote plasma unit.
 10. The methodof claim 1, wherein the second phase comprises generating a plasma fromnitrogen gas alone, and supplying the excited nitrogen species from theplasma.
 11. The method of claim 1, wherein the second phase comprisesgenerating a plasma from nitrogen gas and hydrogen gas, and supplyingthe excited nitrogen species from the plasma.
 12. The method of claim 1,wherein the layer of dielectric silicon nitride has an index ofrefraction greater than about 1.9.
 13. The method of claim 1, whereinthe layer of dielectric silicon nitride has an etch rate of less thanabout 1.5 nm/min in a buffered, 7:1 dilute HF solution.
 14. A method ofdepositing a SiN film on a substrate in a reaction space, the methodcomprising: contacting the substrate with a silicon precursor; andsubsequently contacting the substrate with excited nitrogen species,wherein the residence time of the excited nitrogen species in thereaction space is less than about 1.0 s, and repeating contacting thesubstrate with the silicon precursor and the excited nitrogen species toform a film consisting essentially of dielectric SiN.
 15. The method ofclaim 14, further comprising purging the reaction space betweencontacting the substrate with the silicon precursor and contacting thesubstrate with the excited nitrogen species.
 16. The method of claim 14,wherein the excited nitrogen species is supplied to or formed in thereaction space for greater than about 0.1 s.
 17. The method of claim 14,wherein the excited nitrogen species are supplied from a remote plasmaunit.
 18. The method of claim 14, wherein the silicon precursorcomprises a silicon amine.
 19. The method of claim 14, wherein thedeposited SiN film has an index of refraction greater than about 1.9.20. The method of claim 14, wherein the deposited SiN film has an etchrate of less than about 1.5 nm/min in a buffered, 7:1 dilute HFsolution.